I'd say do straight stitching through the light parts (diagonally) and then also through the corners, so the quilting is all diagonal boxes. That should be enough, and easy enough to do USING YOUR PRESSER FOOT!
Start in the middle (longest run) and roll up the excess that will be in the inside of your machine. Then unroll as you go. Then repeat on the other side, turn the quilt and go the opposite direction.